This invention generally relates to mechanical sealing assemblies. In particular, the invention relates to a mechanical seal assembly for use on a rotating shaft, and designed to provide strong sealing capabilities with low maintenance and long wear-life.
Mechanical seal assemblies are well known in the field of mixing and other rotating equipment, and the difficulties and challenges of a strong, low maintenance seal construction are also well known. In the usual application, the mechanical seal is subject to substantial friction wear, thermal degradation, and corrosion. In addition, mechanical seal assemblies are often called upon to provide a vapor or pressure seal such as, for example, mixing applications for the food or chemical industries. In these situations, thermal degradation, friction, corrosion or shaft wear can create more pronounced problems since even a modest degradation of the seal surface could result in the loss of pressure or a vacuum loss and could provide a means for contamination.
In most constructions, the mechanical seal is mounted to the housing at the point in which the rotating shaft exits the housing. Friction from the point of contact between the housing, the shaft and the seal assembly is thus concentrated in one or more finite areas, and these areas become maintenance focal points. To accomplish the actual seal, disposable packing material is often interposed within the seal assembly to sustain the friction and thermal degradation. As a result of such wear and abrasion, the disposable packing material must be periodically replaced. The equipment must usually be taken out of service during packing replacement and often the shaft, housing, or other attachments must be removed in order to accomplish the packing replacement.
With many conventional sealing assemblies, a common problem experienced is that since stationary packing material is applied directly to the rotating shaft, continued rotation of the shaft serves to wear away or compress the packing material or its housing. As a result, a path for contamination develops and a pressure tight environment inside the mixing or pumping housing cannot be maintained. Various prior art devices have addressed this problem with varying degrees of success. In U.S. Pat. No. 5,571,268 by Azibert, various means are used to direct pressure inward toward the shaft to maintain a seal including use of a plastic strap with handles (FIG. 11). In U.S. Pat. No. 5,509,664 by Borkiewicz, a plurality of mated circumferential seal segments are arranged about the rotating shaft, and these seal segments are subjected to a circumferential force by means of compression coil springs.
Other disadvantages often featured in prior art devices include the feature that often the sealing means is achieved with an elastomeric O-ring or other gasket material that functions at the point of contact between the rotating and stationary members. While a more pliable member is helpful to form a pressure and vapor-tight seal, such construction is not optimum because a pliable medium located at the precise point of contact between the stationary and rotating surfaces deteriorates quickly, resulting in substantial maintenance and inspection efforts.
Another serious disadvantage of many conventional seal assemblies found in the prior art is that they are ill equipped for use on a rotating shaft that has been damaged by many years of use. It is not uncommon in many process industries to find rotating shaft equipment that is fifty or more years old with shaft surfaces that are worn and pitted due to heavy usage. With many conventional seal assemblies, the members are constructed such that a pressure tight seal cannot be achieved on an imperfect shaft. In fact, the nature of many conventional seals is such that they greatly contribute to the degradation of the shaft surface in the vicinity of stationary housing, leading to considerable down time and expense in replacing or repairing the shaft.
Another problem that often compromises the usefulness of many prior art devices is that if the shaft is bent or the bearings worn, the shaft will not turn true relative to the housing. This problem is known as "runout", and it creates a sealing problem for many prior art devices that are dependant on a stable perpendicular alignment between the shaft and the housing.
Some of the problems associated with replacement of mechanical seals can be overcome through the use of a split seal assembly. Split seal constructions are well known in the art and provide the advantage that removal of the shaft or motor housing is normally not required since the split mechanical components can be concentrically disposed around the shaft without removal. In many split seal constructions, the seal consists of at least two sealing rings either axially spaced from each other or adjacent to each other. In most constructions, one seal ring is mounted or attached to the shaft and rotates with the shaft while the other is mounted or biased to a stationary housing.
One example of a fully split mechanical seal assembly can be found in U.S. Pat. No. 5,662,340 by Bessette et al. The Bessette device provides inherent advantages over many seals in the prior art, yet still features the disadvantage that the direct point of sealing contact is directly upon the shaft. Therefore, this device does not address the goal of protecting the shaft from inordinant wear. Furthermore, the sealing properties may be compromised if the rotating shaft in question features a pitted or imperfect surface.
U.S. Pat. No. 5,403,020 to McOnie discloses a split seal device which features a vulcanized rubber insert which engages a fractured seal ring to the rotating shaft. A series of additional fractured rings and other rotary bodies are attached to and between the rubber insert and a stationary body to form the seal. The McOnie device purports to be inexpensive and easy to manufacture, and that the diametrically opposed, uneven fractured lines of the ring members aid in preventing leaks. A minimum number of the parts are subject to wear resulting in reduced maintenance costs. However, the McOnie device features a large number of parts, and the nature of the device is such that the end of the rotary shaft must be fully accessible for the initial installation. Applicability to existing shafts, especially one with an imperfect surface, could be questionable.